1. Field of the Invention
The present invention relates to a system for driving the rotary component of a fan, a pump or the like by supplying power from a variable frequency power source (referred to as "VF power source" hereinafter) and more particularly to a VF power source operating system for operating a VF power source under an abnormal condition in which VF power source control signals can not be sent to the VF power source due to disconnection accident or short-circuit accident in the signal line.
2. Description of the Prior Art
A conventional VF power source operating system of this kind is shown in FIG. 1, in which indicated at 1 is a commercial power line, at 2 is a switch, at 3 is a VF power source for controlling the speed of a motor 4, at 6 is the rotary component of a fan, a pump or the like, at 5 is a mechanical coupler for coupling the motor 4 and the rotary component 6, at 7 is a controller which gives a frequency control signal to the VF power source 3, at 8 is a control command given to the controller 7 and at 9 is a signal line interconnecting the controller 7 and the VF power source 3. In FIGS. 2 and 3, t.sub.1 and t.sub.3 are a time point when an increase control command is given and a time point when a decrease control command is given respectively, t.sub.2 and t.sub.4 are time points when the frequency of the output of the VF power source reaches the increased and decreased target frequencies, respectively, t.sub.5 is a time point when an accident occurred in the signal line 9 and t.sub.6 is a time point when the frequency of the output of the VF power source 3 reaches the frequency corresponding to the absence of the signal on line 9.
The functions of this conventional VF power source operating system will be described hereinafter. In order to give a concrete description, the rotary component 6 is regarded as a fan for supplying its output (air) to the boiler, not shown, of a power plant.
Referring to FIG. 1, the VF power source 3 receives power through the switch 3 from commercial power line 1 and provides an output for driving the motor 4. The revolving rate n of the motor 4 is represented by the following expression: EQU n=(120.times.F)/P (1)
where F is the frequency of the output power of the power source and P is the number of poles of the motor.
Accordingly, the revolving rate n is proportional to the frequency F of the output power of the power source, that is, the revolving rate n of the motor 4 varies according to the variation of the output frequency F of the VF power source 3.
The motor 4 is coupled with the fan 6 with a coupler 5 and the fan 6 supplies air to the boiler at an output air supply rate Q which is approximately proportional to the revolving rate n of the motor 4.
In a power plant, the air demand of the boiler changes as the power demand of the associated power grid changes or the fuel supply mode of the boiler changes. In such a case, a control command 8 to change the air supply rate is given to the controller 7 and a control signal for deciding the air supply rate Q is given through the signal line 9 to the VF power source 3, and then the VF power source 3 provides an output of a frequency F corresponding to the required air supply rate Q.
FIG. 2 is a time chart for explaining the normal actions of the conventional VF power source system shown in FIG. 1. In FIG. 2, when the control command 8 requests the increase of the air supply rate Q at a time point t.sub.1 and the decrease of the increase air supply rate to the original air supply rate at a time point t.sub.3, the output of the controller 7 is given through the signal line 9 to the VF power source 3. Ordinarily, the VF power source 3 is designed to respond to the request for the change of the output at a fixed increase rate or a fixed decrease rate, therefore the change of the output frequency F of the VF power source 3 is delayed slightly from the time points t.sub.1 and t.sub.3 of request for the change and the output frequency changing action of the VF power source 3 is completed at the time points t.sub.2 and t.sub.4. That is, if it is desired to change the output air supply rate Q of the fan 6, the revolving rate n of the motor 4 is changed by changing the output frequency F of the VF power source 3.
FIG. 3 is a time chart showing the changes of controlled factors in case the disconnection of the signal line 9 including the loosening or the falling off of the connecting terminal, by way of example, has occurred. In this case, naturally, there is no change in the control command 8 and the control command 8 is requesting a fixed air supply rate Q. When the disconnection of the signal line 9 occurred at a time point t.sub.5, the input signal of the VF power source 3 becomes zero. Consequently, as explained in connection with FIG. 2, the output frequency F of the VF power source decreases with a slight delay to a lower limit value causing the decrease of the revolving rate n of the motor 4, and thereby the output air supply rate Q is decreased.
Since the operation of a motor by means of a conventional VF power source has been performed in the manner as described hereinbefore, the output frequency of the VF power source 3 varies unnecessarily in case an accident occurs in the control signal to be given to the VF power source 3, which entails unstable operation of the motor 4 and the rotary component 6 and the resulting accident in the system.